Multiple-input and multiple-output (“MIMO”) is the use of multiple antennas at both a transmitter (e.g., a base Station “BS” and/or user equipment “UE”, otherwise referred to as a mobile device) and a receiver (e.g., a BS and/or UE) in order to improve wireless communication between the transmitter and the receiver. Massive MIMO, otherwise referred to as a MaMi network, refers to using a large number of antennas (e.g., equal to or greater than a threshold number) at the transmitter, receiver or both. For example a base station may have hundreds of antennas, arranged in an antenna array, while the user equipment will have at least one and, optionally, two to four antennas.
In order to calibrate the antennas in a MaMi network and focus energy to the UEs from the antennas, thereby maximizing antenna gain, the UEs transmit a pilot signal, in a dedicated time slot, which is listened for by all the antennas at the BS. In response to receiving the pilot signal, the BSs will record details of the RF signal (e.g., reflections, delays, phase, etc.) as a means of reading the wireless channel (e.g., determining delays, echoes and the like in the wireless channel) on which the UE is communicating. The BS uses this information to configure the BS's antennas in the digital domain and provide the antennas with appropriate signals (i.e., signals having appropriate, amplitude, phase, reflections and the like). As a result when the BS transmits signals to the UE, the BS only transmits signals (and listens for signals) in the direction from where the BS received signals. In the MaMi network this results in high antenna gain, i.e., multiple directional beams being transmitted simultaneously with the appropriate delays, which results in focused energy at the UE and optimal polarization at the UE.
The validity of the pilot signal that is transmitted from the UE is very time limited (i.e., time coherency is minimal). If the UE is moved a short distance the pilot signal will no longer be valid and the channel will appear different. Therefore, the pilot signal needs to be transmitted on the uplink frequently (e.g., once every millisecond (ms) or the like). The benefit of such frequent transmission is that the RF energy improves, approximately, by a factor of 100 and the spectral efficiency improves, approximately, by a factor of 10. In other words, the same frequency/channel can be re-used for up to 10 UEs simultaneously.
If the pilot signal is not being received properly by the antennas at the BS, due to disturbance, interference or the like, the BS is incapable of calculating the channels and otherwise optimizing the wireless network. For example, what is referred to herein as pilot contamination or interference can result from a UE (i.e., the contaminated UE) transmitting their pilot signal during their designated time slot, while another UE is transmitting on the same frequency/channel at the same time (i.e., simultaneously on the same pilot channel). When this occurs the BS is unable to differentiate between the two signals.
Therefore, a need exists to improve performance of the uplink pilot signal, so that better overall MaMi network performance can be realized in the form of improved channel estimate. In this regard, a need exists to address problems associated with pilot contamination, specifically, addressing the need to keep inter-cell interference low while, at the same time, keeping the spectral efficiency high.